[Exim] Under which circumstances Exim sends an automatically…

Hello,

today I saw in the logfile a message like this

2002-12-09 14:54:16 12002-12-09 14:54:16 18LOMR-0002Vn-00 =>
xxxx.xxxx@??? <xxxx.xxxx@???> R=smart_route T=remote_smtp
H=merkur.hrz.uni-giessen.de [134.176.2.12]

I found no corresponding incomming message, only this single outgoing
one.
There was no messages in the queue for longer then 20 min and the
receipient of this message was no local user.

Under which circumstances Exim sends an automatically generated message
to an extern user?

Maybe an impotant hint:
As a result of an misconfiguration there was no space left on the root
partition at that time the message was send.
But I don't think that this have caused the message.
(/var/spool/mail is mounted on it's own partition.)

It was only this one message.
Any hint?

oliver